The manufacture of abrasive tools employing a monolayer of superabrasive particles is a relatively old art which has become increasingly important over the last two decades as such tools have found new applications as well as replaced more traditional metal, vitreous, and resin bonded multi-layered abrasive tools. Superabrasives are understood by those skilled in the art to mean synthetic or natural diamond, cubic boron nitride and any similar very hard abrasive materials.
Historically, monolayer abrasive tools used for heavier duty applications in the market place were primarily of electroplate construction utilizing mechanical entrapment of the abrasive particles until the early to mid-1970's when a brazing method was successfully introduced which braze bonded diamonds to a tool substrate using a hard, high strength alloy such as the nickel, chromium, boron containing alloy described in U.S. Pat. No. 3,894,673.
Since that time other brazing type processes for monolayer abrasive tools using strong, high melting point alloys have been described, however, to date none have achieved as wide spread acceptance as the tools described in the above referenced U.S. patent.
In the decades between the 1950's and 1970's, it was found that relatively low melting point and softer braze alloys, such as the silver/copper eutectic and near eutectic composition, in the presence of a relatively small amount of titanium or zirconium would readily wet diamond. This led to a recommendation of using such alloys to form monolayer diamond wheels using such brazes. However, subsequent experience in the field proved these alloys failed to form a bond exhibiting adequate mechanical properties which would retain the diamond particles under the rigors of grinding and cutting applications for commercial acceptance. Tools made using these teachings could not compete with the electroplated versions and were never acceptable by the industry. Therefore the use of such soft brazes have been essentially directed to less demanding non-abrasive tool application, such as the bonding of a single diamond crystal to a substrate for mounting a phonographic needle, or electronic semiconductor applications. More generally such alloys are promoted for bonding ceramics to ceramics or metals in other non-abrasive tool applications.
While it was recognized by many skilled in the art that the lower melting point braze alloys, such as mentioned above, exhibited many of the more desirable characteristics for braze bonding superabrasives, such as diamond, the mechanical failure of these softer alloys in the fabrication of commercially acceptable monolayer diamond tools led others to try to develop a process using stronger braze alloys which may adequately wet diamond and resist swarf abrasion to form a strong enough bond for commercially satisfactory acceptance. This led to the teachings of using a hard, strong nickel or cobalt braze as disclosed in U.S. Pat. No. 3,894,673.
The commercial success of the strong brazing alloy disclosed in the aforementioned U.S. Pat. No. 3,894,673 encouraged others skilled in the art to attempt to achieve an alternate brazing method using a high strength, hard alloy such as the molybdenum/iron alloy method disclosed in U.S. Pat. No. 4,968,326 which reportedly has some actual commercial use.
One of the drawbacks of the brazing process disclosed in U.S. Pat. No. 3,894,673 and others using the harder alloy systems is their relatively high melting point and significant difference in coefficient of thermal expansion between diamond and the alloy which introduces certain stresses upon the diamond crystals which are not relieved to the same extent as in the softer, lower melting point brazes. Further, such alloys do not readily wet cubic boron nitride particles without a metallic coating. The use of such nickel or cobalt based alloys also required the use of the high grade, less friable diamond particles which are better able to resist the stresses inherent in such processes to diminish instances of premature diamond crystal failure to an acceptable level.
An alternate more recently taken approach by a major U.S. manufacturer has used the near eutectic or eutectic silver/copper braze alloy with a titanium content for wetting to braze a monolayer superabrasive tool; however, they employ an electroless overplate of nickel-phosphorus in an attempt to overcome the known lack of mechanical strength of this relatively soft alloy. The electroless nickel overplate adds cost and further appears not to have improved the performance of such an abrasive tool which has been reported as still not possessing wholly satisfactory characteristics for abrasive tool applications. These characteristics include satisfactory cutting rates and sufficiently long useful life.
While many of those skilled in the art have attempted to improve upon existing technology directed to construction of a brazed monolayer abrasive tool over the last few decades, prior to the present invention, these attempts have failed to provide a commercially satisfactory abrasive tool having a monolayer of superabrasive particles which is useful for a wide variety of abrasive applications using solely a relatively low melting point braze alloy as the bonding medium.